AUGUST 29, I9I =| 
Sought the question 4 the influence of ae 
upon rainfall has not received much attention in 
this country, but on the Continent,—in France, 
Germany, and Austria especially,—in America, 
and, more recently, in India, the arguments for and 
against the existence of any influence have been 
put forward at great length, and sometimes with 
much energy. 
The literature on the subject is somewhat 
bewildering, not only on account of its extent, 
but also because of the surprising divergence of 
views entertained by different authorities. Most 
European and some American writers are in favour 
of the accuracy of the supposed forest influence, 
while other American authorities maintain that the 
effect is entirely fictitious; that the instrumental 
records which have been adduced in support of it 
are affected by errors brought about by differences 
of exposure in the forest and in the open, and 
that, if there is a connection between forests and 
climate, it is the latter which controls the growth 
of the former, the former having no appreciable 
effect on the climate. 
On one side the problem has been attacked by 
the historical method; that is, the state of a forest 
and the amount of rainfall in its vicinity are com- 
pared together over as long a period as possible. 
On account of the lack of trustworthy records of 
rainfall for the long periods required, the fall is 
usually estimated from accounts of the condition of 
some stream or river in the neighbourhood. As an 
example of this method may be cited the case of 
the river Loire, which in former times afforded 
communication by water between Nantes and the 
central provinces of France. In 1551 the Marquis 
of Northampton went from Orleans to Nantes, 
with his suite, in “five large, many-cabined boats,” 
whereas navigation is now impossible above 
Saumur, the distance of which from Nantes is 
less than half that of Orleans. This change is 
ascribed to the deforestation carried on extensively 
in the surrounding country in the seventeenth 
century, and the consequent diminution in the 
volume of water in the Loire due to diminished 
rainfall. It is here tacitly assumed that the 
general climate over Western Europe has remained 
unaltered throughout the period, and that any 
change in the climatic conditions is due to local 
forest influence, secular changes of climate being 
entirely overlooked. 
The strongest arguments in favour of the sup- 
posed influence are based upon observations at 
so-called “parallel’’ stations; i.e. meteorological 
stations are established within a forest area and 
in the open country round the forest, respectively, 
and a long series of simultaneous observations are 
made at all the stations. In nearly all localities 
where such observations have been carried out, 
appreciable difference exists between the rainfall 
measured inside the forest and that measured out- 
side, the forest station having an excess of pre- 
cipitation over the “parallel” station. A remark- 
able example is that of Lintzel, in Hanover. In 
1882 the rainfall at this station was considerably 
less than at most neighbouring stations. Young 
NO. 2235, VOL. 89] 
NATURE 
an | 
| fore 
_ effect of forests on rainfall. 
663 
trees were planted round the station in 1877, ana 
as they grew up the rainfall at Lintzel gradually 
increased in comparison with its neighbours, until 
in 1890 it was generally in excess where i in 1882 
it had been in defect. The objection urged against 
the historical method does not apply to this kind 
of reasoning, which appears conclusive on the face 
of it. In a series of recent papers in the 
\Meteorologische Zeitschrift, J. Schubert has shown 
that a forest station in West Prussia and Posen 
has from 2 to 10 per cent., and in Silesia from 2 to 
6 per cent. more rainfall than a parallel station in 
the open country. From this it is argued that 
inasmuch as a forest increases the rainfall over 
its Own area, it may be expected to produce some 
effect of the same kind in the surrounding districts, 
because the wind would carry forward the rain- 
bearing clouds formed by the forest influence. 
The value of observations derived from parallel 
stations has, however, been strongly criticised by 
some American meteorologists. Prof. Cleveland 
Abbe has urged that the results are vitiated on 
account of the fact that a rain gauge exposed in 
a forest clearing is not subjected to winds as strong 
as those which pass over a gauge at a parallel 
station in the open country; and that, in conse- 
quence, the forest gauge may be expected to record 
more rain, although the real fall may be identical 
at the two places. As a result of his investiga- 
tions, Abbe is of opinion that there is no appre- 
ciable difference in the rainfall outside and inside 
a forest. Schubert was aware of the force of this 
contention, and definite allowance was made for 
difference of exposure in the results quoted above. 
His margin is, however, so small, and the correc- 
tion allowed on account of exposure differences is so 
uncertain, that his final result cannot be regarded 
as furnishing a conclusive solution of the problem. 
In a report on the “Influence of Forests on 
Climate and on Floods,” Prof. Willis L. Moore, 
Chief of the United States Weather Bureau, brings 
forward some considerations against the supposed 
One piece of evidence 
shows how climate affects the extent of a forest 
| area, and suggests that the influence, if any, is 
from climate to forest, and not conversely. Mr. 
E. Huntington, travelling in Chinese Turkestan, 
stated that “poplar forests, which once extended 
for scores of miles, now form wastes of branchless 
dead trunks, like ‘gaunt grey skeletons, and beds 
of dead reeds cover hundreds of square miles. It 
has often been asserted that the destruction of 
forests has been the cause of the diminution of 
rainfall. In the Lop basin the opposite appears 
to be the case; the supply of water has diminished, 
and therefore the forests have died.” 
The physical explanation for the increased rain- 
fall which is put forward is that the evidence is 
undisputed that air temperature is less and per- 
centage humidity is greater over a forest than 
over the neighbourhood. In favourable circum- 
stances, condensation of water vapour may there- 
be set up over a forest, and once the 
condensation has started, it may continue auto- 
matically, owing to the large amount of latent heat 
